1. Field of the Invention
The present invention relates to a sheet detecting apparatus for detecting the sheet conveyed and an image forming apparatus having the same.
2. Description of the Related Art
In general, a sheet conveying portion of an image forming apparatus includes a sheet detecting apparatus capable of detecting a front end position of the sheet in order to match the timing for sending the sheet to a transfer position and the timing for sending the image formed in the image forming portion to the transfer position (see U.S. Pat. No. 6,011,948).
Here, FIGS. 26 and 27A to 27C illustrate the sheet detecting apparatus of the related art. As illustrated in FIG. 26, the sheet detecting apparatus of the related art is provided on the downstream side in the sheet conveying direction of a pair of conveying rollers 618 and 619 closest to the transfer position at which the image formed in the image forming portion is transferred. The sheet detecting apparatus includes a lever member 623 that abuts on the sheet, an optical sensor 624, a light-shielding flag 625 for shielding an optical path starting from the light-emitting portion of the optical sensor 624 and ending at the light-receiving portion, and a stopper portion 626 for positioning the lever member 623 at the home position. The lever member 623 is rotatably formed by a rotational axis 627 and is adapted to return to the home position by a pressing force of a return spring 628 even when it rotates. The light-shielding flag 625 is formed integrally with the lever member 623 and rotates along with the lever member 623.
As illustrated in FIG. 27A, if a front end of the sheet S is brought into contact with the lever member 623, the lever member 623 rotates in the direction of the arrow of FIG. 27A with respect to the rotational axis 627 from the home position, and the light-shielding flag 625 blocks off the optical path of the optical sensor 624. If the optical sensor 624 detects that the optical path is blocked, the sheet detecting apparatus recognizes that the front end of the sheet S has arrived at the lever member 623. Then, the sheet S moves while being in contact with the front end of the lever member 623. If the rear end of the sheet S is separated from the lever member 623, the lever member 623 rotates in the direction of the arrow of FIG. 27C by the return spring 628 and returns to the home position. At this moment, the light-shielding flag 625 retracts from the optical path, and the light-receiving portion of the optical sensor 624 receives the light emitted from the light-emitting portion again so that the sheet detecting apparatus recognizes that the rear end of the sheet S has passed through the lever member 623.
However, in recent years, users demand still higher throughput for the image forming apparatus. In order to improve the throughput in the image forming apparatus, it is necessary to improve the conveying speed of the sheet or to shorten the distance (hereinafter, referred to as a “sheet interval”) between the rear end of the preceding sheet and the front end of the subsequent sheet. Therefore, it is necessary for the sheet detecting apparatus to return the lever member 623 to the home position within a period corresponding to a short sheet interval after the preceding sheet S has passed.
Meanwhile, as the front end of the sheet S having passed a pair of conveying rollers 618 and 619 abuts on an abutting portion, the lever member 623 of the related art rotates by being pressed by the sheet S. As the rear end of sheet S is separated from the abutting portion, the lever member 623 is reversely rotated and returns to the home position. For this reason, a distance necessary as the sheet interval becomes the sum of the distances D1 and D2, in which D1 denotes a distance between the position where the rear end of the preceding sheet passes through the abutting portion of the lever member 623 and the home position where the front end of the subsequent sheet abuts on the abutting portion, and D2 denotes a distance at which the subsequent sheet is conveyed therebetween (refer to FIG. 27B).
Here, the distance D2 is set to Δt×v, in which Δt denotes the time taken for the lever member 623 to move by the distance D1, and v denotes the sheet conveying speed. When the lever member 623 performs a reciprocating movement, the distance D1 for returning the lever member 623 to the home position is generated, and distance D2 at which the subsequent sheet S is conveyed in the return operation is lengthened as the sheet conveying speed increases. For this reason, the sheet detecting apparatus of the related art has a problem in that the distance of sheet interval becomes longer as the conveying speed of the sheet S becomes faster. This suppresses further improvement in throughput.
In this regard, the invention provides a sheet detecting apparatus capable of improving the throughput by suppressing the sheet interval from being lengthened even when the sheet conveying speed increases and an image forming apparatus having the same.